This invention relates to a new method for improved surveying of a bore hole using a magnetic survey device. More particularly, this invention relates to a method for determination of the azimuth of a bore hole.
Where magnetic survey instruments are used in the determination of the azimuth of a bore hole, it is the normal practice to relate the magnetic information to the Earth's magnetic field in the determination of the measurement azimuth of the bore hole. The problem is getting accurate magnetic surveys when in close proximity to an adjacent well due to the magnetic interference from magnetic steel components like liners, casing, fish etc., of an adjacent well hole. These components distort the measured magnetic field (TMF(x,y,z)) leading to errors in the calculated azimuth of the bore hole. The magnetic interference field (IB(x,y,z))coming from the adjacent well is derived from a series of magnetic poles of unknown strength and location. If the location of the poles and/or their effect on certain axes of the magnetic instrument were known then the true Earth's field and accurate surveys could be derived. However, it is more common for the magnetism in the adjacent well's tubular not to be known, neither the pole strengths nor their location. In this instance, this new invention allows an improved azimuth of the well being drilled to be derived.